The present invention relates to a two-stroke cycle diesel engine, and, more particularly, to a two-stroke cycle diesel engine adapted for use as a small-size automobile engine.
A two-stroke cycle engine has theoretically the advantage that an engine of a certain size can generate a greater power than a four-stroke cycle engine of an equivalent size because the two-stroke cycle engine has twice as many work cycles per revolution as the four-stroke cycle engine. However, with regard to conventional small-size diesel engines, in practice, power output per unit effective stroke volume of a two-stroke cycle engine is not substantially different from that of a four-stroke cycle engine. This is due to the fact that scavenging is insufficient in small-size two-stroke cycle diesel engines and power output per unit effective volume per combustion stroke is very low. In fact, a volumetric efficiency as high as 80% is available in four-stroke cycle engines, while on the other hand the volumetric efficiency of the typical two-stroke cycle engine is still as low as 40-50%. Conventional small-size two-stroke cycle diesel engines mostly depend upon crankcase compression for the compression of scavenging air. However, since the pump stroke volume of crankcase compression is equal to the stroke volume of the engine, and since the crankcase has a relatively large clearance volume, the compression ratio of crankcase compression is relatively low, so that as a result the amount of air drawn into the crankcase is small, the amount of delivered air is small, the delivery pressure is low and hence the scavenging pressure is low. Consequently it is hard to supply a really adequate amount of scavenging air into the power cylinder. As a result, the delivery ratio obtained in an engine wherein scavenging is effected only by the normal crankcase compression is only as high as 0.5-0.8. Since further the trapping efficiency is about 0.7, the volumetric efficiency becomes as low as 40-50%, as mentioned above.
The purpose of scavenging is to push the residual exhaust gases in the power cylinder out of it by fresh air, and, therefore, if the pressure of the residual exhaust gases and the distance between the scavenging port and the exhaust port are given, the time required for completing scavenging is determined by the pressure and the amount of scavenging mixture, provided that stratified scavenging is performed, wherein the amount of scavenging mixture determines how strongly the initial supply of scavenging mixture is backed up. If the scavenging pressure is low, as when crankcase compression is used, a relatively long time is required for completing scavenging, particularly when the scavenging is performed by uniflow scavenging, and therefore, when the engine is rotating at high speed, it may well occur that the exhaust port is closed before the scavenging is completed, so that a large amount of exhaust gas still remains in the power cylinder, and thereby only a very small amount of fresh air is charged into the power cylinder. Therefore, conventional two-stroke cycle engines have been unable to operate satisfactorily in the high-speed range.
Furthermore, when scavenging depends only upon crankcase compression, since a power piston also operates as a pump piston, as a matter of course the operational phase difference between a power cylinder-piston assembly and a pump cylinder-piston assembly is exactly 180.degree.. Therefore, the pump piston of a pump cylinder-piston assembly just reaches its top dead center (TDC) when the power piston of a power cylinder-piston assembly reaches its bottom dead center (BDC). In this connection, in the present description the TDC of a piston means the dead center of the piston at the end of the compression stroke of the piston, while the BDC of a piston means the dead center of the piston at the end of the suction or expansion stroke of the piston. In this case, however, although a half of the scavenging period is still left in the power cylinder-piston assembly when its power piston has reached its BDC, the pump piston now begins the move towards its BDC, whereby the pressure in the crankcase rapidly lowers after the power piston traverses its BDC, so far as to generate a partial vacuum in the crankcase, thereby causing the problem that the scavenging period is not all effectively utilized.
In view of the aforementioned problem of poor engine performance due to insufficient scavenging, we have proposed, in U.S. Pat. No. 4,185,596 and co-pending U.S. patent application Ser. No. 917,244, to improve the performance of two-stroke cycle gasoline engines by assisting or by replacing the conventional scavenging dependent only upon crankcase compression with scavenging dependent upon a reciprocating type pump cylinder-piston assembly which is separate from and is driven by the power cylinder-piston assembly in synchronization therewith. This substantially increases the pressure and the amount of scavenging mixture when compared with the conventional scavenging dependent only upon crankcase compression. By combining substantially increased pressure and amount of scavenging mixture with a power cylinder-piston assembly incorporating uniflow scavenging, the volumetric efficiency of the power cylinder-piston assembly is substantially increased. To further improve the performance of such a two-stroke cycle gasoline engine, the operational phase of the separate pump cylinder-piston assembly relative to that of the power cylinder-piston assembly is shifted so that the top dead center of the pump cylinder-piston assembly is behind the bottom dead center of the power cylinder-piston assembly by a certain phase angle. This phase relationship extends scavenging to the period after the BDC of the power cylinder-piston assembly increasing the volumetric efficiency of scavenging of the power cylinder-piston assembly.
The abovementioned particular combination of scavenging by substantially increased pressure and amount of scavenging mixture available from a separate pump cylinder-piston assembly and the uniflow scavenging of a power cylinder-piston assembly depends upon the idea, confirmed by experiments, that it is possible to push the exhaust gases existing in the power cylinder uniformly out of it by the scavenging mixture at high pressure without causing any detrimental mixing between the scavenging mixture and the exhaust gases. In this case, if the amount of scavenging mixture is increased so as to be necessary and sufficient, and if the duration of scavenging is long enough, scavenging at high scavenging efficiency is accomplished, and, as a result, the volumetric efficiency increases, resulting in corresponding increase of engine output power. By contrast, if the scavenging pressure is increased in cross or in loop scavenging, the flow of scavenging mixture is liable to penetrate through the layer of exhaust gases existing in the power cylinder in a short-cutting manner, and scavenging mixture and exhaust gases may also be mixed with each other, thereby not only causing poor scavenging but also increasing blow-out loss of mixture, thus lowering the volumetric efficiency.
Further, in the former applications it has been proposed that the two-stroke cycle gasoline engine incorporating uniflow scavenging should have a two-stroke cycle power cylinder-piston assembly having two horizontally opposed pistons. This is related with the fact that the engine is particularly intended for use with automobiles. As described in detail in the specifications of the aforementioned former applications, currently there exists a great demand for the development of cars which have low fuel consumption, in view of energy saving. Cars also must satisfy a high standard with regard to the prevention of air pollution. In order to improve fuel consumption, not only improvement of the fuel consumption of the engine itself but also reduction of the air resistance of the vehicle are required.
We have noted, in connection with various running tests carried out to prepare for the qualification tests for conforming to the standard for the prevention of air pollution (which are becoming more severe nowadays), that fuel consumption is different in summer and in winter due to the difference of atmospheric air density, and we more keenly recognized that the air resistance of the vehicle has an important effect on the fuel consumption of the vehicle even in low speed running. In order to lower the air resistance of the vehicle, it is important to reduce the height of the vehicle as much as possible and to form the vehicle in a streamlined external shape. Particularly, it is very effective to lower the engine hood. In order to reduce the height of the vehicle, it is effective to eliminate the driving shaft for driving the rear wheel so that the shaft tunnel is eliminated and the entire floor may be flat, thereby constructing a vehicle body having a low floor and a low roof. A method for accomplishing this is to employ the FF system, i.e. the front engine-front drive system. In order to lower the engine hood by a large amount in an automobile of the FF type while ensuring necessary legroom for the driver and the front seat passenger, it is necessary to reduce substantially the height and length of the entire engine compartment. Furthermore, in order to reduce the air resistance of the vehicle, it goes without saying that the frontal area of the vehicle must be reduced. Therefore, the width of the vehicle should be minimized. Furthermore, since the transmission, differential gears, and other driving mechanisms must be housed in the engine compartment together with the engine, in the FF system, the space allowed for the engine is much reduced. Light trucks are often designed with the engine mounted under the driver's seat, and in such a design the engine, being relatively long, often extends so far backwards as to make a hump due to the engine enclosure rearward of the cabin, thus shortening the deck. Depending upon the recognition of these facts, the desire for obtaining an engine which is low in its height, short in its length, and yet not very large in its width, and which has relatively high output power when compared with its volume was combined with the idea of substantially increasing pressure and volume of scavenging mixture in a two-stroke cycle engine incorporating uniflow scavenging so as to increase substantially the performance of the two-stroke cycle engine, resulting in the choice of a two-stroke cycle power piston assembly having two horizontally oppposed pistons and incorporating uniflow scavenging.
In the aforementioned former applications, the above-mentioned problems and solutions have been dealt with as problems and solutions related with gasoline engines. However, these problems and solutions are also pertinent to the task of constructing a small-size two-stroke cycle diesel engine suitable for use with automobiles. In other words, it is possible to improve performance of a small-size two-stroke cycle diesel engine suitable for use with automobiles by employing the structures proposed in the former applications. Therefore, a feature of the two-stroke cycle diesel engine of the present invention is also the combination of scavenging by high pressure and large amount of scavenging air available from a separate pump cylinder-piston assembly and uniflow scavenging of a pump cylinder-piston assembly having two horizontally opposed pistons.
A small-size high speed diesel engine for automobile purposes requires a strong swirl flow of scavenging air in order to supply sufficient amount of air to each particle of fuel injected into the power cylinder and to avoid smoking. The swirling flow of air must be more intensified as the rotational speed of the engine increases. Since automobile engines are operated in a wide speed range, it is required that the intensity of the swirling air flow should be changed in a wide range, because if the swirling of the air flow is too weak the fuel particles are not supplied with sufficient amount of air, so that longer time is required for combustion and smoking is caused, while on the other hand if the swirling of the air flow is too strong, such as to be higher than 100 m/sec, a larger amount of heat is lost through the wall of the cylinder so that ignition timing is retarded and diesel knock increases.
In order to give stronger swirl to the intake air in accordance with increase of rotational speed, conventional small size high speed diesel engines for automobiles are generally formed with a vortex chamber at a part of the cylinder chamber so that the air which flows from the main chamber space of the cylinder to the swirl chamber generates strong swirl flow in the swirl chamber, and fuel is injected into the swirl flow generated in the swirl chamber. In an engine having such a swirl chamber, stronger swirl flow is obtained as the rotational speed of the engine increases. However, an engine having an auxiliary chamber such as a swirl chamber has the drawbacks that the loss of effective energy due to throttling loss is relatively large and that heat loss is large due to increase of cooling surface of the combustion chamber caused by the provision of an auxiliary chamber, thereby lowering mean effective pressure and increasing fuel consumption.